Merge pull request #1348 from tschaub/beforechange

Add `ol.ObjectEvent` for changes to `ol.Object` properties.  Previously, `ol.Object` dispatched instances of `goog.events.Event` with type `change`.  Now `ol.ObjectEvent` instances will be dispatched on property changes.  The events include a `getKey` method to get the name of the property being changed.  The `beforepropertychange` type event is fired before a property value changes, and the `propertychange` type event fires after the property value changes.

src/ol/layer/layergroup.js

@@ -10,6 +10,7 @@ goog.require('ol.Collection');
 goog.require('ol.CollectionEvent');
 goog.require('ol.CollectionEventType');
 goog.require('ol.Object');
+goog.require('ol.ObjectEventType');
 goog.require('ol.layer.Base');
 goog.require('ol.source.State');
 
@@ -104,7 +105,8 @@ ol.layer.Group.prototype.handleLayersChanged_ = function(event) {
     for (i = 0, ii = layersArray.length; i < ii; i++) {
       layer = layersArray[i];
       this.listenerKeys_[goog.getUid(layer).toString()] =
-          goog.events.listen(layer, goog.events.EventType.CHANGE,
+          goog.events.listen(layer,
+              [ol.ObjectEventType.PROPERTYCHANGE, goog.events.EventType.CHANGE],
               this.handleLayerChange_, false, this);
     }
   }
@@ -120,8 +122,8 @@ ol.layer.Group.prototype.handleLayersChanged_ = function(event) {
 ol.layer.Group.prototype.handleLayersAdd_ = function(collectionEvent) {
   var layer = /** @type {ol.layer.Base} */ (collectionEvent.getElement());
   this.listenerKeys_[goog.getUid(layer).toString()] = goog.events.listen(
-      layer, goog.events.EventType.CHANGE, this.handleLayerChange_, false,
-      this);
+      layer, [ol.ObjectEventType.PROPERTYCHANGE, goog.events.EventType.CHANGE],
+      this.handleLayerChange_, false, this);
   this.dispatchChangeEvent();
 };